In theory, bipolar batteries can be used to improve battery energy storage capacity on a weight and volume basis, to reduce packing weight and volume, to provide stable battery performance and low internal resistance.
A bipolar battery construction comprises an electrically conductive bipolar layer, so called biplate, that serves as electrical interconnection between adjacent cells in the battery as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the biplate must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell.
These requirements are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the biplate and in alkaline batteries due to the creep nature of electrolyte. Achieving the proper combination of these characteristics has proven very difficult. For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that are structurally poor for containment of gases present and generated during cell operation. In a sealed construction, gases generated during charging need to be chemically recombined within the cell for stable operation. The pressure-containment requirement creates additional challenges in the design of a stable bipolar configuration.
Battery manufacturers have not developed bipolar batteries commercially because a working seal design has always been a problem. The vast majority of development work to date has been strictly related to lead/acid technology. The seal is difficult to achieve due to the galvanic creepage of the electrolyte, the corrosive conditions and the heat and pressure generated by the battery. Other manufacturers have tried to make leak-proof seals, and use rigid approaches that ultimately fail due to thermal expansion and pressure changes. In the subject disclosure, the pressure created in each battery cell may be vented through a pressure vessel if the pressure exceeds a predetermined level.
New requirements in the field of transportation, communications, medical and power tools are generating specifications that existing batteries cannot meet. These include higher cycle life and the need for rapid and efficient recharges.
NiMH systems are seen as the alternative to meet cycle life, but costs for existing conventional fabrication are too high.
In the recharge of an ideal battery, the energy would be stored with 100% efficiency, and the recharge should terminate when the 100% state was reached. in most batteries, this can be determined by knowing the relationship between battery temperature and the desired final voltage. Because batteries are not 100% efficient, it may require 104% (for new lead acid batteries) to achieve 100% recharge. Lead acid batteries exhibit a fairly sharp increase in cell voltage as they approach full charge. Power supplies can be set to sense a voltage, and terminate the charge at that point. Alternatively, when a set voltage is reached, the power supply can be programmed to supply a limited additional amount of charge. Nickel Cadmium and Nickel Metal Hydride cells have a different characteristic: when approaching full charge, the voltage begins to decrease.
Charge voltage is always higher than the cell open circuit voltage, because it must overcome resistive losses, which are additive to the required voltage to recharge the cell. The amount of higher voltage is proportional to the rate of recharge. Nickel cells accept current at elevated temperatures at lower voltages. The problem arises when trying to recharge fully, or recharge electrodes that are not uniformly discharged.
The portions of the electrodes with lower resistance or that are fully charged will begin overcharging before the rest of the cell is charged. These areas will convert the charge energy to oxygen via electrolysis. The oxygen then recombines on the negative electrode producing an equivalent amount of heat, and the temperature of the cell increases. The temperature increase will be greater in the area that is overcharged and recombining, so increasing amounts of the recharge will flow through the warmer areas. While this may damage the cell in time, it prevents the ability to determine that a battery is fully charged based upon its voltage.
In U.S. Pat No. 5,344,723 by Bronoel et al., a bipolar battery is disclose having a common gas chamber, which is created by providing an opening through the biplate (conductive support/separator). The opening is also provided with a hydrophobic barrier to prevent passage of electrolyte through the hole. Although the problem with pressure differences between the cells is solved, there are still a disadvantage with the described battery. The outer sealing around the edge of each biplate still has to be fluid-tight, which is very difficult to achieve. If the outer sealing is not fluid-tight, the electrolyte, contained in the separator between the electrodes, may migrate from one cell to another.
There is a need for a battery that is easy to manufacture at affordable prices, and that are safe to handle during charge and discharge procedures.